Radiation sensitive V79 Chinese hamster cells, selected after a ouabain challenge, were found to be sensitive to the usual freeze (liquid N2)/thaw cycle employed for storing mammalian cells. In contrast to parental cells with which recoveries of 75-100 percent are obtained, fewer than one percent of many sensitive clonal isolates were recovered as viable, colony forming cells. Treatment after X-radiation with medium containing 90% D2O led to the expression of potentially lethal damage. Similarities were observed with postirradiation treatment with anisotonic buffer in respect to both cell killing and the repair of DNA damage (single-strand breaks expressible at high pH).